The invention relates to a dual-mode surface acoustic wave filter (DMS filter) that is preferably produced on a LiNbO3 or LiTaO3 base. The DMS filter has acoustic tracks that are coupled to one another via coupling converters. Interdigital converters are provided for each of the tracks and are disposed between reflectors. The DMS filter can be used, in particular, for RF applications.
In communication systems, signals are processed balanced or unbalanced, with frequent changes being made between two operating modes for carrying signals and thus resulting in a requirement for filters which, considered on the input and output sides, can be operated unbalanced/balanced or balanced/balanced. Specific circuit concepts also require filters with the characteristics indicated above, as well as different input and output impedances.
A DMS filter which to some extent satisfies these requirements, that is to say can be operated balanced or unbalanced, is known in the prior art. The structure of this DMS surface acoustic wave filter has acoustic tracks which are coupled to one another via coupling converters with interdigital converters which are disposed for each track within short-circuited reflectors and act as filter input and output converters, and are connected in series. Where filters of this type have been produced, they have in each case the same impedance, namely 50 ohms, on both sides, that is to say on the filter input and output sides.
It is accordingly an object of the invention to provide a dual-mode surface acoustic wave filter which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a dual-mode surface acoustic wave filter which can be operated balanced and unbalanced, containing: coustic tracks, including:
coupling converters for coupling the acoustic tracks to one another;
reflectors; and
interdigital converters disposed between the reflectors and act as input/output converters for each of the acoustic tracks, the input/output converters have an impedance determined by splitting the input/output converters into a plurality of individual converters, and at least some of the individual converters are connected in series for each of the acoustic tracks.
The object of the invention is to provide a highly selective RF filter, which can likewise be operated balanced or unbalanced (symmetrical or unsymmetrical), that is to say has so-called BALUN functionality, but in which case the impedance of the input and output converters may optionally be defined to be different, in contrast to the known DMS filter.
In order to achieve this object, the invention provides that the impedance of the input and/or output converters of the filter is determined by weighting and/or by splitting the input and/or output converters into a plurality of individual converters, at least some of which are connected in series, for each acoustic track.
Some of the input and/or output converters may in this case also be split into individual converters, which are connected in parallel.
By changing, for example, the geometric structure of uniform normal finger converters, it is always possible to set a desired impedance for the individual converter and a different impedance from converter to converter, which may be obtained, by omission weighting or by overlap weighting and, if required, in combination with this, by splitting the converters into a plurality of series-connected individual converters.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a dual-mode surface acoustic wave filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.